下面代码，无论that放哪里，怎么整都提示：fcanvasToTempFilePath: fail canvas is empty，真是无语，麻烦求解，谢谢。 onReady: function ()   {     var that = this;     var context = wx.createCanvasContext('mycanvas',that);     wx.chooseImage     ({       success: function (res)       {         context.drawImage(res.tempFilePaths[0], 0, 0, 150, 100);         context.fillStyle = "#FF0000";         context.fillRect(0, 0, 50, 50);         context.draw(true, setTimeout(function()         {           wx.canvasToTempFilePath           ({             destWidth: 150,             destHeight: 100,             canvasId: 'mycanvas',             success: function (res)             {               console.log('1212' + res.tempFilePath);             },             fail: function (res) {console.log(res);}           },that);         },300));       }     });   }